Digital cameras offer significant advantages over scanners for capturing document images and other images. For example, digital cameras are generally more portable than scanners. In addition, because scanners require that the document to be digitized must be placed on a scanner platen, digital cameras are capable of capturing images of a wider variety of document types (e.g., larger documents or documents mounted in a frame) than scanners. However, the use of digital cameras creates difficulties that do not exist when using a scanner. For example, lighting conditions vary when using a digital camera, whereas the lighting conditions are generally controlled for scanners. In addition, use of a digital camera can introduce geometric distortions, which can depend on various factors such as the angle and distance of the digital camera relative to the document, the lens used by the digital camera, and whether the document is situated on a flat or curved surface. Because scanners generally utilize a moving scanner head, at a fixed distance from a document which is held flat on the platen, these distortions do not generally occur in scanners. Another difficulty in capturing images of documents with a digital camera is the wide variety of different possible background shapes, patterns and colors.
U.S. Pat. No. 8,326,015 to Nepomniachtchi, entitled “Methods for mobile image capture and processing of documents,” discloses a method for document image capture and processing using mobile devices. An image of a financial document is captured using a mobile communication device and processed for data extraction. The processing includes automatically detecting the document within the image, geometrically correcting the image, binarizing the image, correcting the orientation of the image, and correcting the size of the image. The process of locating the document within the captured image uses color reduction (convert to gray scale), and morphological operations to suppress noise. “Voting” points are identified that represent the edges of the document and approximate a straight line from those points. The document corners are determined by finding the intersection points of the straight lines.
U.S. Pat. No. 8,000,514 to Nepomniachtchi et al., entitled “Methods for mobile image capture and processing of checks,” and related discloses a method for document image capture and processing using mobile devices. An image of a check is captured using a mobile communication device and processed for data extraction. This patent teaches document location in the same manner as the aforementioned U.S. Pat. No. 8,326,015. The captured check images are binarized and processed using perspective correction based on the determined document location.
U.S. Pat. No. 7,978,900 to Nepomniachtchi et al., entitled “Systems for mobile image capture and processing of checks,” discloses a method for automated document processing using mobile devices. Captured images are optimized and enhanced for data extraction. The processing operations include automatically detecting the document within the image, geometrically correcting the image, binarizing the image, correcting the orientation of the image, and correcting the size of the image. An analogous process is described in U.S. Pat. No. 7,953,268 to Nepomniachtchi, entitled “Methods for mobile image capture and processing of documents.”
U.S. Pat. No. 8,340,437 to Abràmoff et al., entitled “Methods and systems for determining optimal features for classifying patterns or objects in images,” discloses a method for determining features for classifying patterns or objects. The described method includes separating color channels and using Independent Component Analysis for object detection for medical imaging.
U.S. Patent Application Publication No. 2003/0156201 to Zhang, entitled “Systems and methods for processing a digitally captured image,” discloses a method for processing a digitally captured image of a document. The method includes transforming the digitally captured image into a grayscale image and applying an adaptive threshold to determine a binary image. The binary image is searched using iterative pixel progression to detect a plurality of edges. The detected plurality of edges are analyzed to determine at corners associated with the imaged document so that the identified corners can be used for perspective correction.
U.S. Pat. No. 7,835,589 to Heaney, Jr. et al., entitled “Photographic document imaging system,” describes a method for processing a captured image including a document. The method includes distinguishing an imaged document from its background, adjusting the captured image to reduce distortions and properly orienting the document. Edges of an imaged document are identified using an edge detection algorithm, and intersection points of the lines are determined to be potential corners.
U.S. Pat. No. 8,320,683 to Konishi, entitled “Image processing method, image processing apparatus, image reading apparatus, and image forming apparatus,” describes a method for comparing an obtained document image against a set of references images and determining which reference images is most like the obtained reference image.
There remains a need for improved methods for processing captured images of documents to identify a document boundary that can be used to perform perspective correction.